The invention relates to a process for the preparation of perfluorocarboxylic acids of the formula RFxe2x80x94COOH, salts thereof and esters thereof from perfluoroalkyl iodides of the formula RFxe2x80x2xe2x80x94I by oxidation with oxygen under the action of light, metals, free-radical and RFxe2x80x2 can be cyclic, branched or linear, saturated or unsaturated perfluoroalkyl radicals.
Perfluorocarboxylic acids are compounds with extremely high chemical and thermal stability and excellent surface-active properties. There is a great need for perfluorocarboxylic acids, sometimes in high purity, for a multitude of different applications. Their excellent emulsifier properties are utilized, for example, during the emulsion polymerization of fluorinated olefins, such as, for example, tetrafluoroethylene. For example, DE-A-3512633 describes the use of alkali metal and ammonium salts of perfluorooctanoic acid as emulsifier in the preparation of shear-stable PTFE copolymer dispersions.
A number of processes for the preparation of perfluorocarboxylic acids are already known, but they all have serious disadvantages.
For example, perfluorocarboxylic acids can be obtained by hydrolysis of perfluorocarboxylic acid halides, which are obtained from the corresponding fluorine-free The handling of liquid hydrogen fluoride, the yield which decreases considerably with increasing chain length and the formation of chain-branched rearrangement products limit the applicability of this process.
DE-A-3606174 describes the oxidation of perfluoroalkylethylene of the formula F(CF2)nxe2x80x94CHxe2x95x90CH2 with permanganate to give perfluorocarboxylic acids. This process has the disadvantage that relatively large amounts of manganese oxides are produced as waste material. The ozone described in U.S. Pat. No. 4,138,417 as oxidizing agent for perfluoroalkylethylene is not practicable on an industrial scale either.
DE-A-2756169 describes the reaction of perfluoroalkyl iodide F(CF2)nxe2x80x94I with zinc or with activated zinc and carbon dioxide to give perfluorocarboxylic acids. The low yield, the need for certain solvents and the consumption of zinc are disadvantages of this process.
DE-A-3043249 describes the oxidation of perfluoroalkyl iodide F(CF2)nxe2x80x94I with fuming sulfuric acid at temperatures of 100-180xc2x0 C. The difficult handling of fuming sulfuric acid and the high requirements placed on the chemical resistance of the apparatus present considerable difficulties when this process is carried out on an industrial scale.
We have now found a process which permits the preparation of perfluorocarboxylic acids in high purity from readily accessible perfluoroalkyl iodide under mild reaction conditions and with the avoidance of relatively large amounts of waste products. Surprisingly, it has been found that perfluoroalkyl iodides in organic solvent with the action of light or other activating substances or conditions in the presence of oxygen, with even atmospheric oxygen sufficing, can be converted to perfluorocarboxylic acids of high purity in high yields even at room temperature. Suitable perfluoroalkyl iodides are cyclic, branched or linear, saturated or unsaturated compounds. Examples of such compounds are F(CF2)ixe2x80x94I, (CF3)2CFxe2x80x94(CF)jxe2x80x94I, F(CF2)kxe2x80x94CF(CF3)xe2x80x94I, F(CF2)lxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94I, F(CF2xe2x80x94CF(CF3))mxe2x80x94I and Fxe2x80x94(CF2xe2x80x94CF2)n(CF2xe2x80x94CF(CF3))pxe2x80x94I, where i, j, k, l, m, n and p are each integers from 0 to 20. Preferred organic solvents are, in particular, lower alcohols such as methanol and ethanol. As well as light, suitable activators for perfluoroalkyl iodide are also relatively high temperatures, metal catalysis or free-radical sources, such as, for example, free-radical initiators.